1. Field of the Invention
This invention relates to radiography. In a primary application the invention relates to the encoding of information representing different x-ray energies onto photographic film where the decoded information is used to selectively image specific materials.
2. Description of the Prior Art
Conventional radiography records the transmission through the body over a broad spectrum of x-ray photon energies. This integration over the energy spectra results in a superposition of all tissues and materials in the final image making it different to visualize many structures of interest. For example, tumors and other soft tissue lesions are often obscured by bone. Also, large amounts of iodine must be administered to blood vessels, in highly invasive procedures, to make them visible in the presence of other tissues.
Different materials have attenuation coefficients which have unique functions of energy. By making measurements at different regions of the energy spectrum, and combining them in appropriate data-processing systems, specific materials can be removed or isolated. A general system with this capability is described in U.S. Pat. No. 3,848,130, "Selective Material X-Ray Imaging System," issued to A. Macovski. Here the measurements made at various energy spectra are combined to make images representing different materials. In this patent, however, the images of the various measurements made separately and in sequence. As a result, a registration problem exists when the various images are combined.
The registration problem is solved by the system described in U.S. Pat. No. 3,950,613, "X-Ray Encoding and Decoding System," issued to A. Macovski. In this patent the x-ray energy information is encoded on a single film in the form of repetitive grating patterns. These grating patterns are formed by x-ray absorbing gratings which are placed between the x-ray source and film-screen cassette. Although this solves the registration problem by placing all the measurement images on a single film, it introduces resolution problems. The x-ray encoding gratings must have relatively low spatial frequencies in order to be adequately resolved by the scintillating screen. The information encoded using this low-frequency grating has relatively poor resolution. In addition, x-ray encoding gratings are difficult to fabricate and, in some configuration, necessitate additional radiation dose to the patient.
A preferred approach to the use of energy-selective information in radiography is described in U.S. Pat. No. 4,029,963, "X-Ray Spectral Decomposition Imaging System," issued to R. E. Alvarez and A. Macovski. Here measurements are made at two regions in the x-ray energy spectrum, with the measurements processed to form the photoelectric and Compton scattering components of the materials. These components, representing essentially atomic number and density, can be combined to represent different materials. This patent described both the CT and projection radiography applications of selective energy imaging.
In the projection system a single-exposure system is described using an energy-selective dual-cassette detector where the lower energies are absorbed primarily in the front screen and the higher energies in the back screen. These resultant images are recorded selectively on a single color film by having the front and back screens produce different colors. These are then scanned to provide the desired low-energy and high-energy information. The use of color film, however, presents significant difficulties. The film is both expensive and less sensitive than black and white film. Also, the use of screens emitting different colors restricts the choice of optimum screen material.